Geothermal Energy

Lea Rekow

This background text aims to inform readers about the basics of geothermal energy in general, Iceland’s geothermal energy sector in particular, and the outlook for harnessing geothermal energy internationally.

Our planet’s crust is composed of hard rock, broken up into several gigantic tectonic plates that make up the upper 20 km or so of the Earth’s surface. The heat from the Earth’s core causes convection currents in the semi-solid mantle just below the crust to push these tectonic plates around like toys in a bathtub. During tectonic plate separation, some of the tremendous heat from deep within the Earth comes to the surface.

As the Eurasian and North American plates below Iceland move apart, a plume of mantle material upwells from deep within the Earth. Temperatures in other parts of the world typically increase by 35℃/95°F for every kilometer below the surface, but in Iceland they increase much more rapidly, soaring to over 200℃/392°F at just one kilometer below the surface. This geology powers Iceland’s volcanoes and other geothermal activity.

Geothermal energy is the transfer of heat from the earth’s interior to the atmosphere. Heat warms water that has seeped into underground reservoirs. These can be accessed by drilling, or at the tectonic plate boundaries, the water can independently break through the Earth’s surface as hot water or steam. Whichever way it is accessed, the steam can be directed into a turbine to generate electricity. After use, the cold water is pushed back into the Earth where it’s reheated to produce steam again, in a repeating cycle.

Iceland has a functionally inexhaustible supply of geothermal energy and has invested in harvesting it on a national scale. The country’s energy transition began in earnest in the 1970s during the global oil crisis. During this time, Iceland halted almost all oil imports, and accelerated its plans to implement geothermal heating. Iceland’s geothermal is indeed a success story in many ways, but one that has created new problems for the island nation. Geothermal use has expanded very rapidly, both in the form of district heating and in power generation.

Approximately 25-30 percent of all of Iceland’s electricity comes from geothermal sources. The remainder is almost exclusively derived from hydropower. Iceland produces substantially less expensive electricity per kWh than many of its European counterparts. Around 80 percent of this energy, mostly enabled by hydropower, provides for a heavy industries sector that is attracted to Iceland because of the low price point companies pay for electricity. The rest is directed to domestic use, for the most part, district heating generated by geothermal.

Iceland has built seven major geothermal power stations. The electricity is generated by driving turbines to rotate electrical generators, in much the same way as traditional fossil fuel or nuclear power plants operate. A flash steam system is typically used, whereby bore holes are drilled to access water that is super-heated by the geothermal temperatures. This water expands into steam and pushes up through pipes to drive a turbine. Some of the water cools and condenses and is sent back down to a geothermal reservoir in a closed loop system to reheat, and the cycle is repeated.

District heating is either derived from geothermal power plants, or naturally heated water that surfaces in the form of natural pools or geysers and seeps back into the Earth in a continuous cycle. This thermally heated water is pumped all over the island via a network of heavily insulated pipes to provide 85-90 percent of all district heating for homes and businesses. It is also piped under roads and pavements to eliminate snow accumulation. There is so much spare hot water in Iceland that it can be pumped into freezing lakes to keep parts warm enough for fish to survive throughout the coldest months of winter.

Geothermal is also used to heat greenhouses, allowing for fresh produce to be grown year-round. For example, a typical commercial greenhouse can produce around one ton of tomatoes a day using a combination of geothermal heating and electricity. To maintain ideal growing conditions, greenhouses are often powered by small-scale, on-site geothermal turbines, used in conjunction with several kilometers of heated water pipes installed at ground level. The electricity powers grow lights to enhance or enable photosynthesis and thus crop yields, while the heating pipes are used to maintain temperatures. The pipes may also function as rails to transport trolleys back and forth between aisles. Harnessing this type of decentralized geothermal to produce food allows for Iceland to achieve greater domestic food security by more sustainable means. Produce harvested in the morning can be on market shelves by the afternoon. This reduces the emissions, pollution, and the expense associated with the international transportation of imported produce.

In Iceland, where the heat is available closer to the surface, horizontal flash steam loops can be created by digging as little as 6-8 feet deep, or vertical flash steam loops by digging down around 250-300 feet deep. Both can meet small on-site needs. High-capacity power plants that serve local areas might drill as far down as 3 km. In Iceland’s largest and most experimental geothermal project, however, the Hellisheiði plant has drilled down 4.7 km into a volcano to access water that is neither gas nor liquid, but rather takes the form of a supercritical fluid.¹

The massive Hellisheiði geothermal power plant, located just 30 km from Reykjavik, is one of the world’s largest.² It is owned and operated by Orka Náttúrunnar (ON) a subsidiary of Iceland’s publicly owned Reykjavík Energy company, Orkuveita Reykjavíkur (OR). A small portion of the energy produced here is used for district heating, however its almost exclusive purpose is to supply electricity for nearby aluminum refineries, in particular the Norðurál aluminum smelter owned by Century Aluminum, a publicly traded North American-based aluminum producer whose parent organization is the mining and commodities giant, Glencore Plc.³

The Hellisheiði plant began operations in 2006 and has since undergone two rapid expansions, one in 2008 and the other in 2011. Sulfur pollution and environmental health impacts have increased significantly with each expansion, to affect residents of the nearby town of Hveragerði, located only ten km away. Here, residents endure increased incidences of respiratory problems, coughing, and nausea to such an extent that they have been encouraged by at least one physician to take magnesium and iodide supplements to counteract the health impacts from the power plant’s hydrogen sulfide (H2S) pollution.⁴

Monitoring has revealed that H2S levels in the atmosphere in and around Hveragerði regularly reach levels that should place the town inside the plant’s dilution area, where residential housing is not permitted. Permanent residence is prohibited inside dilution areas by law and land use is significantly restricted. According to the World Health Organization (WHO), Iceland’s geothermal dilution area regulations are weak by international standards, yet even these weak standards are not enforced. Dilution areas for geothermal plants in general need to be much larger than for other polluting industries. However, in the case of the Hellisheiði power plant, the dilution area has yet to be defined even after years of operation.

Hellisheiði’s H2S pollution reaches as far as Reykjavík at levels that often exceed both Icelandic and international safety standards.⁵ A link between the Hellisheiði plant’s sulfur pollution and an increased need for asthma medication in the greater Reykjavík area suggests that particulates are causing environmental health problems here as well.⁶

Reinjection of geothermal fluids at the Hellisheiði plant also cause regular earthquakes reaching up to level 4 on the Richter scale.⁷ On a particularly seismically active day in September 2011, for example,1,500 earthquakes were felt in Hveragerði. The geothermal pumping system has also leaked wastewater to the surface. This has resulted in a series of unplanned effluent lagoons forming near the plant. Lakes like these can pose a serious threat to freshwater systems as elevated levels of arsenic, mercury, lithium, and boron are likely to be present in geothermal fluids.⁸ These chemicals negatively impact the surrounding environment, especially if waste leaches or is released into waterways instead of being reinjected back into the geothermal field. However, as pollutant migration from reinjection sites is not traced, it is not possible to determine what the negative consequences from reinjection are either.

Contamination by chemicals found in geothermal fluids, including bioaccumulators such as mercury, arsenic, and boron, negatively affect aquatic life, drinking water and irrigation sources and may result in:

• Local loss of fish species;
• Local loss of invertebrate species;
• Excess nutrients leading to algal blooms;
• Increased turbidity and decreased water clarity;
• Decreased dissolved oxygen levels;
• Damage to species caused by repeated exposure to sub-lethal doses of contaminants that cause physiological and behavioral changes in species populations to have long term effects such as reduced reproductive success, abandonment of nests and broods, decreased immunity, tumors and lesions, central nervous system impairment, and a decreased ability to evade predators;
• Bioaccumulators, such as mercury, may be absorbed into animal tissues and transferred to humans if consumed.⁹

Different geothermal plants emit different degrees of chemical emissions. Gas emissions at Hellisheiði are high in H2S, but low in carbon dioxide (CO2).¹¹ Hellisheiði produces only about 5 percent of CO2 compared to what is produced by an average coal plant. In contrast, at the Krafla power plant in North Iceland, CO2 makes up 90-98 percent of its emissions, the rest is in H2S. In Iceland, any project emitting over 30,000 tons of CO2 requires an emissions permit. Geothermal power stations tend to hover just under that figure, however even if operations do exceed that level, Icelandic authorities do not consider geothermal emissions anthropogenic, and thus do not include plant emissions in greenhouse gas inventories.¹²

Though there is ongoing pressure to reduce emissions from geothermal plants, this loophole opens the door for more geothermal plants to be constructed, even though they may be sizeable emitters. Further, most of these plants are built to provide electricity for international heavy industry sectors that are significant emitters in themselves. For example, plans for a large geothermal development in Húsavik—proposed to power a single Alcoa aluminum smelter— would have released 1,300 tons of CO2 per MW into the atmosphere. An average gas-powered plant would produce only slightly more—1,595 tons CO2 per MW. The total CO2 produced by the project would have been almost equivalent to all road transportation in Iceland. The plan was eventually scrapped.¹³

In response to increasing pressure to reduce CO2 emissions from geothermal, a new carbon-fixing industry is emerging. ON’s Carbfix pilot project launched at Hellisheiði in 2012, is attempting to lower CO2 by combining emissions gases with water pumped to the surface and reinjecting the solution into the volcanic basalt below. Natural chemical reactions occur when basalt is exposed to carbon dioxide and water, resulting in a white, chalky mineral which can store carbon. In most rocks, it would take hundreds or even thousands of years to form. However, in the basalt beneath Hellisheidi, it takes less than two years for 95 percent of the CO2 injected to solidify.¹⁴ This means that large amounts of CO2 can be pumped into basalt and stored over a very short period. The project is now expanding both at Hellisheiði¹⁵ and to other geothermal sites, yet it has some very large obstacles to overcome. It is a tremendously complex physical process that is extremely energy intensive and expensive, with sequestration costing between $750-$1,000 per ton.¹⁶ Though there is little economic profit likely to be generated from Carbfix¹⁷, the carbon sequestration business is finding new investment and technology partners.

Several significant gaps in proven best practices have been identified in the 2018 assessment report for the Hellisheiði power plant, including a flawed original assessment of resource capacity and environmental impacts, unforeseen and ongoing expenditure, and relatively low returns on investment.”¹⁸

Other significant gaps in the areas of social and environmental responsibility include:

• A lack of personal and regular interaction with residents in the Hveragerði community;
• A lack of proactive contacts and targeted information dissemination provided to the Hveragerði community;
• No effective process for involving project-affected communities in decision-making on relevant issues;
• Repeated non-compliances in relation to surface releases of geothermal water;
• Governance arrangements that do not support an equitable treatment of municipalities;
• A lack of active promotion of research into H2S exposure-response relationships;
• Uncertainties around positive livelihood outcomes for parts of the community in Hveragerði; and
• Worker dissatisfaction.¹⁹

Despite ecological damage, environmental health risks, and low economic returns, plans for more large-scale geothermal plants continue to be promoted in Iceland’s south, near the Hellisheiði project, as well as in the north in the extremely ecologically sensitive Mývatn district, and on the Reykjanes peninsula. Icelandic energy providers sell electricity at an extremely low price point to the aluminum smelters they power. There is little correlation between the cost of electricity generation and the price of aluminum, yet Iceland’s electricity sales contracts with smelters are based on the price of aluminum. As 2021 and 2022 saw a significant rise in the historical price of aluminum, profits for Icleandic Energy providers, including OR, also increased during this time,^{20 21} though overall there is negligible economic benefit to Icelanders that is generated from these endeavors (either in the form of return on investment, taxes or employment), which makes the argument for supporting large-scale projects, built for the sole purpose of powering the heavy industries sector, more difficult to make.

There are countries other than Iceland that produce substantial amounts of geothermal. The majority of them are situated on or near fault lines where tectonic plates are pushing against one another. The United States is currently the world’s leading producer of geothermal electricity, though it supplies less than a half a percent of the nation’s overall power consumption.

While Indonesia, the Philippines, Turkey, New Zealand, Mexico, Kenya, Italy, and Japan also produce geothermal energy, the world is not yet on track to reach the milestones set out by the International Energy Agency’s (IEA) Sustainable Development Scenario (SDS). The SDS aims to achieve the energy-related components relating to Goals 3 (good health and wellbeing), 7 (affordable clean energy), and 13 (climate action) of the United Nations Sustainable Development Goals.

To meet the IEA’s SDS goals by 2030, geothermal energy production needs to grow globally by more than 10 percent a year, alongside solar, wind, and hydropower. Utilizing geothermal energy on a global scale requires finding feasible ways to access and harness it. As an alternative to the more traditional flash steam system, binary power can be used in countries that aren’t located above fault lines. Using a binary system, hot water from an underground aquifer is pumped into a heat exchanger, where it’s transferred to a lower boiling point fluid. The secondary fluid is converted to vapor to drive the turbine. Water temperature of about 100°C is all that is required for the system to work. Since there’s no deep drilling involved, it can be used by many countries regardless of their location. Nevertheless, access to a sustainable supply of hot water is still required.

A leading geothermal model being promoted to meet large-scale energy needs is the Enhanced Geothermal System (EGS). There are at least 18 major EGS sites that have been developed throughout the European Union, Japan, South Korea, Australia and the USA.²² EGS is created by engineering a subsurface fracture system in which water can be pushed through injection wells to break up rock in order to extract heat. The method is somewhat similar to hydraulic fracturing, even though it’s water that is injected down into the fractures, instead of the toxic chemical and abrasive mixtures that are pumped down the fracking lines. Though it uses much less pressure than hydraulic fracking, EGS is still intended to open up natural rock fractures to create underground reservoirs for water. Geologists and geophysicists have determined that such activity increases the risk of earthquakes. EGS was, for example, linked to a 5.5 magnitude earthquake that occurred in Pohang, South Korea in 2017²³, and plans for a geothermal plant in Switzerland were abandoned after a series of 3.4 magnitude quakes, attributable to drilling, damaged homes in Basel.²⁴

Because of the potential that geothermal offers, there remains motivation to set up the EGS system as it is theoretically possible to place a power plant almost anywhere that can be drilled deep enough to locate hot rock. Thus, there is some optimism that workable solutions to developing this form of geothermal might become more feasible with responsible regulatory oversight. Still, sizeable technical and economic obstacles stand in the way. The drilling process is very expensive, and there is no guarantee that any well will be successful once pressurized water is introduced. Further, there may not be enough decent fractures to make the project economically feasible. Additionally, the presence of minerals like quartz and limestone in existing aquifers may contaminate pumped water. Scaling and fouling caused by these types of minerals can damage pumps and heat exchangers, causing extensive delays that may compromise a project’s economic viability. Due to these difficulties, combined with the steadily falling costs of solar panels, wind turbines, and energy storage over time, there has been more investment in renewable technologies that generate quicker profits for lower cost outlays.

Despite risks and tradeoffs with the EGS method, geothermal is promoted as offering potential environmental, climate, and energy security benefits. In 2015, the International Renewable Energy Agency, a sister organization of the IEA, worked to establish the Global Geothermal Alliance (GGA) at COP 21. The coalition seeks to promote the use of geothermal energy for both power generation and direct heating, while urging governments and businesses to support investments in geothermal energy. By 2030, the GGA hopes to increase geothermal power generation capacity by fivefold and geothermal heating capacity by more than twofold. Theoretically, geothermal operations could be scaled up to meet 3-4 percent of global energy demand by 2050 if serious, long-term investment was made immediately.

Many countries (particularly developing countries) around the world are eager to explore the viability of geothermal technology as an alternative to fossil fuels. Kenya, for example, already derives 46 percent of its total energy needs from geothermal.

The archipelago nation of Indonesia, comprised of thousands of islands that lie over the Ring of Fire, currently generates 5 percent of its energy through geothermal. That could be scaled up to meet as much as 30 percent of its needs. Approximately 29 gigawatts of geothermal power could potentially be accessed via hundreds of locations including on Java, Sulawesi, Sumatra and Bali. That is the annual equivalent output of 29 medium-sized nuclear power plants or 12,344 wind turbines. Currently, 62 percent of Indonesia’s electricity is generated by coal plants that are subject to weak emissions standards. They produce chronic levels of air pollution, which makes geothermal energy a potentially wise option for the country. Cultural and political barriers, however, along with a general lack of public awareness about geothermal, currently inhibit the uptake of this energy form.

In contrast to solar and wind power, geothermal energy’s predictable baseload power is available 24 hours a day, 365 days a year. However, how much geothermal can fulfill a promise to bring increased sustainability to the world’s energy sector remains a hotly contested issue. And if this energy sector can be scaled up, should it, given the environmental costs, the possibility of geologic destabilization, and the health risks from emissions? Finally, if it can be developed as an environmentally friendly technology, how much of it will be utilized to enable carbon-intensive industries to prosper?

It will take time to see how the geothermal industry will mature. Due to the extreme heat, drilling equipment cannot penetrate beyond nine or ten kilometers. Therefore, most geothermal potential will remain untapped for the foreseeable future. There may be 50,000 times more geothermal energy available than the entire global supply of oil and gas combined, but it remains mostly inaccessible.

Even in Iceland, the future of geothermal remains problematic. Geothermal heating is potentially under threat from deglaciation in some places. The extreme force glaciers exert on the underlying land is waning as mass ice loss occurs. As a result, the country’s interior and coastlines are rising by up to an inch and a half a year. Land rise has already warped wastewater pipes in rural towns near the Vatnajökull glacier, to create a host of service delivery problems. If geothermal heating pipes in other places are disturbed as land rises over time, district heating might become compromised in the future.

Moreover, little knowledge is available about the short- and long-term effects of exploiting the heat of volcanic aquifers on a large scale. Drilling and power generation in geothermal areas are difficult to predict because they are incredibly different from one another. The commercial use of geothermal power is not necessarily sustainable or renewable, as wells can dry up quickly or cool down, and take more than a hundred years to recover, while dramatically altering the local environment in the meantime.²⁵ Iceland is famous for its geothermal hot springs yet tapping large-scale geothermal with experimental or underdeveloped technology could result in their irreparable damage or loss. In return, Icelanders receive little economic reward for this energy production, which is sold, often at a loss, to heavy industry. Thus, even though geothermal energy is considered a clean, renewable technology by many, it clearly has particularities that still require solving many complex design, implementation, and environmental management problems through all phases of project planning, development, and utilization, both in Iceland and elsewhere.

LIST OF REFERENCES

¹ Richard, Jeremie, and Gaël Branchereau. “Iceland Drills 4.7 Km down into Volcano to Tap Clean Energy.” Phys.org, Phys.org, 5 May 2017, https://phys.org/news/2017-05-iceland-drills-km-volcano-energy.html.

² Fernández , Lucía. “Largest Geothermal Plants Globally 2021.” Statista, Statista, 8 Feb. 2023, https://www.statista.com/statistics/525206/geothermal-complexes-worldwide-by-size/

³ Glencore, a Swiss trading company, is the world’s largest buyer and seller of commodities. Century Aluminum, 40 percent of which is owned by Glencore (who also has a seat on Century’s board), is a midsize US company that was instrumental in lobbying the Trump Administration to help bring about one of the most dramatic changes in recent trade policy—the aluminum tariffs that started Trump’s Trade War. Century sells most of its aluminum to Glencore, which resells it to buyers through its trading operation (approximately 75 percent of Century’s consolidated net sales come from Glencore). While Century was lobbying the Trump administration to introduce aluminum tariffs, Glencore and a few other commodity trading companies were stockpiling record amounts of foreign aluminum in the US. The theory was if tariffs were announced, foreign metal would become more valuable, and all that cheap metal would be worth more. This is exactly what happened.
For more, see Matthew Philips and Joe Deaux, The Metal That Started Trump’s Trade War, Bloomberg News Businessweek Feature, September 27, 2018. https://www.bloomberg.com/news/features/2018-09-27/the-metal-that-started-trump-s-trade-war?leadSource=uverify%20wall

⁴ “Hellisheiði: A Geothermal Embarrassment.” Saving Iceland, Saving Iceland, 29 Aug. 2012, https://www.savingiceland.org/2012/08/hellisheidi-a-geothermal-embarrassment/.

⁵ Gunnarsson, Ingvi, et al. “Geothermal Gas Emission From Hellisheiði and Nesjavellir Power Plants, Iceland.” GRC Transactions, vol. 37, 2013, https://doi.org/https://publications.mygeoenergynow.org/grc/1030660.pdf.

⁶ Carlsen, Hanne Krage, et al. “Hydrogen Sulfide and Particle Matter Levels Associated with Increased Dispensing of Anti-Asthma Drugs in Iceland’s Capital.” Environmental Research, vol. 113, Feb. 2012, pp. 33–39., https://doi.org/10.1016/j.envres.2011.10.010.

⁷ Halldorsson, Benedikt, et al. 15th World Conference on Earthquake Engineering, Lisbon, 2012, On the Effects of Induced Earthquakes Due to Fluid Injection at Hellisheidi Geothermal Power Plant, Iceland.

⁸ Arsenic concentrations of up to 4.6 ppm have been found in wastewater released from geothermal power plants, yet the maximum recommended safe arsenic level for drinking water is 0.01 ppm, according to the WHO, therefore leaching may pose a significant problem.

⁹ “Chemical Contamination and Geothermal.” NIWA Taihoro Nukurangi, NIWA, 6 Oct. 2020, https://niwa.co.nz/our-science/freshwater/tools/kaitiaki_tools/impacts/chemical-contaminates/causes-of-chemical-contamination/geothermal-energy-and-chemical-contamination.

¹⁰ Mutia, Thecla M., et al. “Concentrations of Sulphur and Trace Elements in Subarctic Soils and Mosses in Relation to Geothermal Power Plants at Hengill, Iceland – Ecological Implications.” Geothermics, vol. 95, Sept. 2021, p. 102136., https://doi.org/10.1016/j.geothermics.2021.102136.

¹¹ Gunnarsson, Hydrogen Sulfide and Particle Matter Levels, 2013

¹² Ármannsson, Halldór, et al. “CO2 Emissions from Geothermal Power Plants and Natural Geothermal Activity in Iceland.” Geothermics, vol. 34, no. 3, 2005, pp. 286–296., https://doi.org/10.1016/j.geothermics.2004.11.005.
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¹³ “No Smelter in Húsavík! – Energy Crisis Force Alcoa to Withdraw.” Saving Iceland, Saving Iceland, 18 Oct. 2011, https://www.savingiceland.org/2011/10/no-smelter-in-husavik-energy-crisis-force-alcoa-to-withdraw/.

¹⁴ “Geothermal Energy: Up-Scaling Geothermal Operations: Hellisheiði & Nesjavellir.” Carbfix, Carbfix Hf., https://www.carbfix.com/geothermalenergy.

¹⁵ “Tenfold Increase to CO2 Direct Air Capture and Storage at Hellisheiði.” ON, ON Orka Náttúrunnar, 5 July 2022, https://www.on.is/en/news/tenfold-increase-to-co2-direct-air-capture-and-storage-at-hellisheidi/.

¹⁶ Beiser, Vince. “The Quest to Trap Carbon in Stone-and Beat Climate Change.” Wired, Conde Nast, 28 Dec. 2021, https://www.wired.com/story/the-quest-to-trap-carbon-in-stone-and-beat-climate-change/.

¹⁷ Ragnheidardottir , Elisabet Vilborg. “Costs, Profitability and Potential Gains of the CarbFix Program.” Reykjavík Energy Graduate School of Sustainable Systems, Reykjavík Energy Graduate School of Sustainable Systems, 2010.

¹⁸ Hartmann, Joerg, and Bernt Rydgren. Orka Náttúrunnar, 2018, Geothermal Sustainability Assessment Protocol: Hellisheidi Geothermal Project, https://www.on.is/wp-content/uploads/2021/03/hellisheidi-assessment-report_final_22-june-2018.pdf

¹⁹ Hartmann, Geothermal Sustainability Assessment Protocol, 2018

²⁰ OR Reykjavik Energy, 2021, Consolidated Financial Statements 2021, https://annualreport2021.or.is/documents/716/OR_Consolidated_Financial_Statements_2021_0BQwDOw.pdf.

²¹ “Average Aluminum Prices from 2012 to March 2022.” Statista, Statista Research Department, 5 May 2022, https://www.statista.com/statistics/276643/aluminum-prices-since-2003/.

²² Lu, Shyi-Min. “A Global Review of Enhanced Geothermal System (EGS).” Renewable and Sustainable Energy Reviews, vol. 81, 2018, pp. 2902–2921., https://doi.org/10.1016/j.rser.2017.06.097.

²³ Zastrow, Mark. “South Korea Accepts Geothermal Plant Probably Caused Destructive Quake.” Nature News, Nature Publishing Group, 22 Mar. 2019, https://www.nature.com/articles/d41586-019-00959-4.
Westaway , Rob. “Evidence Suggests Fracking Linked to South Korea’s 2017 Earthquake.” The Conversation, 31 May 2018, https://theconversation.com/evidence-suggests-fracking-linked-to-south-koreas-2017-earthquake-95883.

²⁴ Gabbatt, Adam. “Swiss Geothermal Power Plan Abandoned after Quakes Hit Basel.” The Guardian, Guardian News and Media, 15 Dec. 2009, https://www.theguardian.com/world/2009/dec/15/swiss-geothermal-power-earthquakes-basel.

²⁵ MacKay, David JC. Sustainable Energy: Without the Hot Air. UIT, 2009.